This invention relates to an organic friction material for facing or lining of the so-called wet system brake or clutch to be used in the presence of an appropriate lubricating oil (cooling oil).
Wet friction materials employed in the prior art include sintered metal type friction materials, paper type friction materials, woven type friction materials, mold type friction materials and rubber type friction materials, and among mold type friction materials, there is a graphite type friction material composed mainly of graphite or coke, and there is also a resin mold type friction material composed mainly of various fillers.
Among these friction materials, sintered metal friction materials have excellent heat resistance as well as high thermal conductivity, and have been employed for heavy duty uses. However, when a friction plate becomes larger in scale, it becomes difficult to make uniformly precise friction plates on account of the restriction in working, whereby partial contact with a mating or opposing plate occurs, and inconveniences are involved such that the friction plate receives concentrated thermal loading in that region which exceed the seizure limit, or undergoes plastic flow.
For this reason, it has been considered to effect dissipation of heat by plastic deforming the friction plate to bring it in contact uniformly with the whole surface of the mating plate. As the friction material for this purpose, there are paper type, rubber type, graphite type and resin mold type, and particularly paper type and rubber type have been increasingly employed.
Paper type friction materials are prepared by the paper making method by adding various additive components to cellulose fibers as the main component and binding them with a phenol resin, and have about 40 to 50% of communicated pores. Since they have a large plastic deformation amount by pressure, they can readily contact with the whole surface of the mating plate, and are deemed to be well compatible. Cellulose fibers have advantages of good affinity for oils and also high frictional coefficient, and hence have been used in large amounts as the material with high power absorption (or rate of energy dissipation) under medium and low energy absorption (or energy dissipation).
However, a drawback of the paper type material is that, since elasticity is obtained through entanglements of fibers, recovery response after pressure deformation is inferior. Also, the fibers are oriented in a certain direction by sheet making, and therefore the surface roughness of the mating plate is required to be made 1 .mu.m or less. Otherwise, there are also problems of stroke adjustment, etc. during pressurization, and fluctuation of frictional coefficient depending on pressure, and the loading capacity is inferior to rubber type.
To improve the paper type material, there is a material in which pulp powder is added in the granulated state as disclosed in Japanese Patent Publication No. 27755/1978, and which material has fibers randomly oriented, thus improving wear resistance and pressure response while maintaining the advantages of the paper type material.
Next, in the case of the rubber type friction material, whole surface contact with the mating plate is obtained through elastic deformation. For this reason, many attempts have been made to use materials enriched in elasticity with low modulus. There have been proposed a friction material using a fluoroelastomer as the binder (Japanese Provisional Patent Publications No. 18749/1977 and No. 85878/1982), a material using an epoxy resin and a nitrile rubber (Japanese Patent Publication No. 2733/1982) and a material using a phenol resin and a nitrile rubber (Japanese Provisional Patent Publication No. 92983/1981). All of these materials have a modulus of elasticity of about 10 to 60 kg/mm.sup.2, have desirable elasticity in the engaged pressures generally employed, and also can be softened by heating even when causing partial contact to occur, thereby enabling treatment energy dissipation through whole surface contact. Thus, under certain use conditions, a material having very high seizing resistance (not heat resistance) has appeared. It is also a specific feature of rubber type that power absorption is very high in uses of medium and low energy absorption.
However, rubber type friction materials have drawbacks depending on the kinds of rubbers, and also common to all of rubber type.
Fluorine rubber type is excellent in heat resistance and oil resistance. However, since fluorine rubber itself has weak strength, expensive materials must be used in large amounts for obtaining necessary material strength, posing a problem with respect to cost. Also, for the same reason, it is difficult to obtain a material with a large amount of porous materials or fillers added therein, and there is also restriction in frictional characteristics.
Rubbers other than fluorine rubber are mostly materials enriched in integrity with resins, and can be polymerized as the binder with other resins or added by mixing with resins. However, these rubbers proved to be not satisfactory in performances for brakes or clutches which are used under severe conditions with respect to heat resistance, oil resistance and thermal aging characteristic. For example, in the case of a material using a carboxy-modified nitrile rubber covalently bound to an epoxy resin as the binder, although very high power absorption can be obtained if energy absorption is small, but under the use conditions where energy absorption is great or the temperature of the whole material becomes higher, such as continuous slip, inconveniences such as buckling or abnormal wear of the material occurs, whereby only very low power absorption could be applied. Also, most of these rubbers have double bonds remaining in the molecule, and also the degree of freedom between molecules is higher as compared with thermosetting resins. Therefore, by the influence of high temperature and oil additives, they undergo decomposition, oxidation, swelling or chemical reactions, whereby various properties such as hardness, elasticity and strength change, thereby involving inconveniences such as marked impairment frictional characteristics such as seizing resistance, friction coefficient and wear.
The friction material by use of a rubber as a part of the binder is prepared generally using a conventional rubber molding method, namely by blending the respective components by means of a kneader such as kneading rolls and Banbury mixer, then applying the blend onto calendering rolls to obtain a sheet, which sheet is placed into a mold to be cured by heating, or charging directly the blend into a mold such as compression transfer molding to be cured therein. However, the porosity of the material is as little as several %, and the cooling effect of the oil can be extended into the inner part of the material only with difficulty, and also troubles may occur in the discharging effect of the oil existing at the interface with the mating material, with concomitant inconveniences such as lowering in friction coefficient depending on the use conditions.
Further, another great drawback of the rubber type is low heat resistance. When a rubber is used as a part of the binder, the material strength becomes generally extremely weak at a temperature of 150.degree. C. or higher, and, at a temperature of 200.degree. C. or higher, most materials are lowered in strength as a matter of course, or may be decomposed even within a short time. For this reason, buckling, abnormal wear and lowering in friction coefficient of the material through compression and shearing force by sliding will occur. Whereas, for heavy duty friction materials for wet system, the material strength under the state elevated to a temperature of 200.degree. to 250.degree. C. is very important even for a relatively short time. Under the use conditions demanded presently for heavy duty materials, the situation where the mating plate, the friction plate and, the lubricant oil as a whole routinely reaches a temperature up to 200.degree. to 250.degree. C. is conceivable, and in fact, does occur in practical applications. However, the lubricant oils frequently used currently have ignition points of 230.degree. to 240.degree. C., and it is not desired for safety that the lubricant oil as a whole is exposed to a temperature near the ignition point, and thus the temperature around this temperature may be considered to be the maximum available temperature under the present situation.
Next, the graphite type or the coke type friction materials are superior in heat resistance but inferior in modulus of elasticity as compared with the rubber type and the paper type, and more suited for high energy absorption but inferior in energy absorption capacity as compared with the rubber type.
Most of the resin mold type friction materials exhibit performances between the paper type and the rubber type, and have relatively higher friction coefficients, and high power absorptions in medium degree of energy absorption. They have high heat resistance, that is, thermosetting resins such as a phenol resin, an epoxy resin, a urea resin, a melamine resin and a polyimide resin generally used for friction materials are also equipped with mechanical elements required during sliding even in use under the temperature conditions of 200.degree. to 250.degree. C. for a short time, and particularly a polyimide resin can withstand temperatures of 250.degree. C. or higher.